This invention relates generally to investment casting and, more particularly, to a binder for making investment casting molds having high green strength and low fired strength.
Investment casting, which has also been called lost wax, lost pattern and precision casting, is used to produce high quality metal articles that meet relatively close dimensional tolerances. Typically, an investment casting is made by first constructing a thin-walled ceramic mold, known as an investment casting shell, into which a molten metal can be introduced.
Shells are usually constructed by first making a facsimile or pattern from a meltable substrate of the metal object to be made by investment casting. Suitable meltable substrates may include, for example, wax, polystyrene or plastic.
Next, a ceramic shell is formed around the pattern. This may be accomplished by dipping the pattern into a slurry containing a mixture of liquid refractory binders such as colloidal silica or ethyl silicate, plus a refractory powder such as quartz, fused silica, zircon, alumina or aluminosilicate and then sieving dry refractory grains onto the freshly dipped pattern. The most commonly used dry refractory grains include quartz, fused silica, zircon, alumina and aluminosilicate.
The steps of dipping the pattern into a refractory slurry and then sieving onto the freshly dipped pattern dry refractory grains may be repeated until the desired thickness of the shell is obtained. However, it is preferable if each coat of slurry and refractory grains is air-dried before subsequent coats are applied.
The shells are built up to a thickness in the range of about xe2x85x9 to about xc2xd of an inch (from about 0.31 to about 1.27 cm). After the final dipping and sieving, the shell is thoroughly air-dried. The shells made by this procedure have been called xe2x80x9cstuccoedxe2x80x9d shells because of the texture of the shell""s surface.
The shell is then heated to at least the melting point of the meltable substrate. In this step, the pattern is melted away leaving only the shell and any residual meltable substrate. The shell is then heated to a temperature high enough to vaporize any residual meltable substrate from the shell. Usually before the shell has cooled from this high temperature heating, the shell is filled with molten metal. Various methods have been used to introduce molten metal into shells including gravity, pressure, vacuum and centrifugal methods. When the molten metal in the casting mold has solidified and cooled sufficiently, the casting may be removed from the shell.
Investment casting molds must withstand significant mechanical and drying stresses during their manufacture. Ceramic shells are designed having high green (air dried) strength to prevent damage during the shell building process. Once the desired mold thickness is achieved, it is dewaxed and preheated to approximately 1800xc2x0 F. At this point, it is removed from the high temperature furnace and immediately filled with liquid (molten) metal. If the mold deforms while the metal is solidifying (or in a plastic state), the casting dimensions will likely be out of specification. To prevent high temperature deformation, molds are designed to have substantial hot strength. Once the casting is solidified and cooled, low fired strength is desired to facilitate the knockout or removal of the ceramic mold from the metal casting.
Most investment casting molds contain significant quantities of silica. The silica usually starts as an amorphous (vitreous) material. Fused silicas and aluminosilicates are the most common mold materials. When exposed to temperatures above approximately 1800xc2x0 F., amorphous silica devitrifies (crystallizes) forming beta cristobalite. Cristobalite has low (alpha) and high (beta) temperature forms. The beta form has a specific gravity very close to that of amorphous silica so mold dimensions remain constant and stresses associated with the phase transformation are minimal. Upon cooling, beta cristobalite transforms to the alpha form. This phase transformation is accompanied by an approximate 4% volume change that creates numerous cracks in the shell, thereby facilitating mold removal. Cristobalite reduces the fired strength of silica containing investment casting molds.
Although investment casting has been known and used for thousands of years, the investment casting market continues to grow as the demand for more intricate and complicated parts increase. Because of the great demand for high-quality, precision castings, there continuously remains a need to develop new ways to make investment casting shells more efficiently, cost-effective and defect-free. For instance, if shell strength was maintained to the point of metal solidification, followed by a reduction in strength as the shell cools, improvements in productivity could be realized through improved knockout (shell removal). This is particularly desirable for alloys of aluminum and magnesium because their melting and pouring temperatures are insufficient to promote cristobalite formation and easy knockout.
Aluminum (and magnesium) castings produced by investment casters are rather fragile, so they are cleaned by water or sand blasting, compared with the aggressive shot blast and vibratory cleaning for steel and high temperature alloy castings. Residual ceramic on steel castings is dissolved away using concentrated acids and bases or molten salt baths. Chemical incompatibility excludes their use on aluminum and magnesium. If a binder was developed having low fired strength and associated easy knockout properties upon exposure to temperatures at or below 1800xc2x0 F., aluminum casting cleanup could be greatly improved.
It is known that cristobalite formation is a temperature and time dependent transformation that is proportional to the reactivity of the silica species. High surface area (small particle) silicas transform at a faster rate than low surface area species. Colloidal silica transforms more quickly than the refractory materials that it binds in the investment casting mold. Additionally, it is reported that certain elements (e.g., sodium) can act as promoters or catalysts for the transformation. The higher the surface area (or smaller the particle diameter) of a colloidal silica, the faster the transformation. Traditionally, colloidal silicas below 7 nanometers have mediocre green strength, low solids concentrations and low fired strengths, making them marginal investment casting binders. Larger particle silicas demonstrate high green strength, but fired strengths can be too high for aluminum applications. If a way of incorporating the best of both colloidal silicas could be found, (sufficient strength for handling and casting, combined with improved knockout) a beneficial investment casting binder would be obtained.
Accordingly, it would be desirable to provide an improved binder for making investment casting molds having high green strength and low fired strength.
The present invention is directed to a binder containing a mixture of colloidal silicas having average particle size diameters of 4, 8 and 13 nanometers. When the inventive binder is used to make investment casting molds, the molds have a high green strength and low fired strength.